The invention relates to a method of measuring the signal quality of an optical data signal, and in particular, to a method of measuring an optical data signal which has data-free time intervals.
In conventional optical transmission systems, and in particular transmission systems operating according to the WDM (Wavelength Division Multiplexing) principle, an optical data signal transmits data as data packets. The data packets may include different types of signals (specifically when WDM technology is used), and multiple transmission channels can be assigned to the individual data packets. These types of signals can differ both in terms of the protocol used and/or the data transmission rates used. Examples of signal types include ATM (Asynchronous Transfer Mode), IP (Internet Protocol) and Gigabit Ethernet.
When monitoring the transmission characteristics of the optical transmission system or of an optical transmission link, the signal quality of the optical data signal currently being transmitted can be determined. In this regard, optical signal characteristic variables in the optical data signal are used to assess the signal quality of the optical data signal. These characteristics may include, for example, the optical signal-to-noise ratio or the signal distortions caused by optical transmission fibers or optical transmitter devices.
In conventional optical transmission systems, such as transmitter devices, optical transmission links are terminated electronically and require an opto-electronic conversion of the data signal.
The signal quality of the optical data signal can thus be determined by the transmitter device with reference to a measurement of the data signal which is opto/electrically converted. As optical transmission systems or transmitter devices are increasingly used, there is a considerable need for methods of measuring the signal quality of optical data signals during an optical data transmission, without the need for a preceding opto/electrical conversion of the data signal which is to be monitored.
Hewlett Packard Product Note 71452-2 xe2x80x9cOptical Spectrum Analyzerxe2x80x9d discloses a method for determining the signal-to-noise ratio of an optical transmission channel in which a measuring instrument is used to transmit an optical measurement signal over an optical transmission link for assessment, the optical measurement signal being interrupted at irregular intervals. When the optical measurement signal is applied, an additional measuring instrument, arranged at the measuring point, is used to determine the signal strength of the optical measurement signal. After an interruption occurs in the optical measurement signal, i.e. in the signal-free time interval, the signal strength of the noise signal on the optical transmission link is determined. The signal-to-noise ratio of the optical transmission link can then be determined by reference to the measured signal strengths. Although the application of this method provides acceptable measurement results, it cannot be used for measuring the signal-to-noise ratio during xe2x80x9clive trafficxe2x80x9d, i.e. during the current data transmission. This method cannot be used because an additional optical auxiliary signal or measurement signal has to be generated and transmitted over the optical transmission link or the optical transmission system.
Accordingly, a need exists for an improved method for measuring the signal quality of an optical data signal.
According to one embodiment of the present invention, a method is provided to measure the signal strength of a transmitted optical data signal. The signal strength of a noise signal is also measured during the data-free time intervals, and the signal-to-noise ratio is determined. The measured signal strengths are used as a criterion for the signal quality of the optical data signal. While the data signal is transmitted and monitored, its signal strength is measured. The optical data signal can then be used as a measurement signal for determining the signal-to-noise ratio of the transmission channel. Since the signal strength of the noise signal is measured during the data-free time intervals, interruptions in the transmission of data which occur in the optical data signal (or in the measurement signal and the data-free time intervals of the optical data signal) can be used to determine the signal strength of the noise signal. Hence, the signal-to-noise ratio of an optical signal element on an optical transmission link, which can be extracted from the optical data signal, can be determined at any desired point without a measurement signal being provided in addition to the data signal. As a result, the signal quality of an optical data signal can be assessed using the signal-to-noise ratio as a criterion, while data is being transmitted.
According to another embodiment of the present invention, a method is provided for selectively inserting additional data-free time intervals into an optical data signal to measure the signal quality. These data-free time intervals may be of the same length and may be inserted into the optical data signal at a predefined time interval. If the optical data signal has too few data-free time intervals to determine the signal-to-noise ratio, additional data-free time intervals may be inserted into the optical data signal. This ensures a requisite measuring accuracy and frequency for determining the signal-to-noise ratio and monitors the signal quality of an optical xe2x80x9cpoint-to-pointxe2x80x9d transmission link.
The present invention also determines a measure of the signal distortion of the optical data signal which is caused by the optical fiber and/or optical transmitter devices, using the ratio of the lengths of the data-free time intervals to the data-transmission time intervals. In addition, to measure the signal strength of the noise signal and of the optical data signal, the chronological lengths of the data-free time intervals and of the data-transmission time intervals (i.e. time intervals in which data are explicitly transmitted) are determined and a ratio is formed between them. The ratio, which is obtained at the site of the measurement, includes, compared to the ratio obtained when the original optical data signal is emitted, an evaluation criterion for the optical signal distortions which are caused by the optical fiber and/or optical transmitter devices.